Add skeletons for function implementations

byg/src/Evaluation/Function.hs

@@ -5,4 +5,12 @@ module Evaluation.Function
 import Types (Function(..), Value(..))
 
 evalFunction :: Function -> Value -> Value
-evalFunction = undefined
+evalFunction f x = case (f, x) of
+  (IsImageFilename, String _) ->
+    Bool undefined
+
+  (ConvertedImageFilename, String _) ->
+    String undefined
+
+  _ ->
+    error "unexpected combination of function and argument type"